Would it be possible to have the Leader annotation type take the <> placeholder and show the calculated curve length as a dimension for a given line? Ideally, this would remain as a live-connection, so it would update the curve length while editing the original curve.

When drawing patterns for softgoods (bags, or even clothing) you'll often want to match the edge length of where parts/patterns meet - they share the same length, but not necessarily the same shape/curve.